The present invention relates to the processing of graphic image files for reproduction by an imaging device such as a color copier, laser printer, image recorder or the like, and more particularly to a method and apparatus for compressing image data to enable the storage thereof in a limited amount of memory provided by an imaging device.
In computer generated image recording, a plurality of computer generated images are rasterized for input to a color copier, laser printer or the like or for exposing corresponding frames of photographic film or another imaging substrate. The copier, printer or film recorder (referred to herein as an "imaging device" or "output device") processes the rasterized image data to provide, e.g., paper copies, transparencies, color slides or motion picture footage containing images represented by the image data.
To obtain the computer generated images, a user first generates a graphic image file using a computer graphics program. The graphic image file may include one or more images or frames of image data which, for example, are in a bit mapped or text (e.g., ASCII) format. The image data are processed by a raster image processor (RIP) which provides frames of raster image data for use by an output device in generating images. The output device may comprise, for example, a color copier, a laser printer, or a photographic film recorder. Film recorders, such as those sold under the Solitaire.RTM. and Sapphire.RTM. trademarks by Management Graphics, Inc. of Minneapolis, Minn., U.S.A. are well known in the art. Color copiers, such as those manufactured by Xerox Corporation and laser printers, such as those manufactured by Hewlett-Packard Corporation are also well known.
In order to image a picture on an output device, all data (which may be in a compressed or uncompressed form) must fit in the available random access memory (RAM) provided by the interface between the graphics workstation and the output device. Thus, in prior art digital imaging systems, the amount of RAM available determined the maximum complexity of a picture that can be imaged. In order to accommodate complex images, a large amount of RAM was required in the interface. Such RAM is expensive, and can substantially increase the price of an imaging system. Thus, a trade-off between the image complexity that can be handled by a particular imaging system and the system's cost has arisen that has required compromises in the types of images that can be handled by lower cost systems.
The present invention substantially overcomes the disadvantage of lower cost prior art imaging systems that are unable to handle images above a certain complexity. This objective is achieved by compressing and, if necessary, successively recompressing rasterized image data with greater loss until the resultant compressed data is able to fit within a RAM budget.